The invention generally relates to implantable cardiac stimulation devices such as pacemakers or implantable cardioverter defibrillators (ICDs) and, in particular, to techniques for predicting the likelihood of a tachyarrhythmia and for controlling overdrive pacing to prevent such an arrhythmia.
An arrhythmia is an abnormal heart rhythm. One example of an arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Other examples of arrhythmias include tachyarrhythmias wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heart beat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, an atrial tachycardia is typically not fatal. However, some tachycardia, particularly ventricular tachycardia, can trigger ventricular fibrillation wherein the heart beats chaotically resulting in little or no net flow of blood from the heart to the brain and other organs. Ventricular tachycardia and ventricular fibrillation, if not terminated, are fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly ventricular tachycardia.
One technique for preventing arrhythmias is to overdrive pace the heart wherein an implantable cardiac stimulation device, such as a pacemaker or ICD, applies electrical pacing pulses to the heart at a rate somewhat faster than the intrinsic heart rate of the patient having a normal sinus rhythm. For bradycardia, the cardiac stimulation device may be programmed to pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heartbeats. To prevent tachyarrhythmias from occurring, the cardiac stimulation device paces the heart at a rate slightly faster than the intrinsic heart rate of the patient.
In one exemplary technique, the cardiac stimulation device monitors the heart of the patient and, if two consecutive intrinsic heartbeats are detected (or two intrinsic beats are detected within a predetermined number of cycles), overdrive pacing is automatically triggered. The overdrive pacing rate is typically set to 5 to 10 ppm higher than the intrinsic rate. The intrinsic heart rate may be determined, for example, by calculating the time interval between the two consecutive intrinsic beats-typically the interval between two consecutive intrinsic P-waves or two consecutive intrinsic R-waves. The stimulation device then overdrive paces the heart at the overdrive pacing rate for a xe2x80x9cdwell periodxe2x80x9d equal to a programmed number of overdrive events or cycles. Thereafter, the stimulation device decreases the overdrive pacing rate by a rate decrement specified by a programmed xe2x80x9crecovery ratexe2x80x9d until additional intrinsic beats are detected, then the device repeats the process to determine a new overdrive pacing rate and paces accordingly.
It is believed that overdrive pacing is effective for at least some patients for preventing the onset of an actual tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as xe2x80x9cectopicxe2x80x9d pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can trigger a tachycardia. By overdrive pacing the heart, the likelihood of the occurrence of ectopic pulses is reduced and the risk of tachycardia may be substantially reduced.
Thus it is desirable within patients prone to tachyarrhythmia to overdrive pace the heart in an attempt to prevent a tachyarrhythmia from occurring. Ideally, overdrive pacing is performed only prior to an expected episode of tachyarrhythmia, rather than continuously. Continuous overdrive pacing is preferably not employed because it may be unpleasant to the patient. The higher heart rate caused by continuous overdrive pacing may also cause damage to the heart or trigger more a serious arrhythmia, such as ventricular fibrillation. Continuous overdrive pacing may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. Continuous overdrive pacing may actually exacerbate heart failure in these patients. Also, continuous overdrive pacing may be a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Moreover, the need to continuously apply overdrive pacing pulses can deplete the power supply of the stimulation device, necessitating surgical replacement of the device on a more frequent basis.
Thus it would be highly desirable to provide a technique for predicting the onset of an episode of tachyarrhythmia so that overdrive pacing can be initiated prior to the tachyarrhythmia in attempt to prevent the tachyarrhythmia from actually occurring. Heretofore, however, no reliable techniques have been developed for predicting the onset of a tachyarrhythmia. Accordingly, it would be desirable to provide a technique for predicting the onset of a tachyarrhythmia, particularly atrial fibrillation, and aspects of the invention are directed to that end. Other aspects of the invention are directed to providing systems and methods for controlling pacing so as to reduce the risk of the tachyarrhythmia.
In accordance with a first aspect of the invention, a technique is provided for predicting the onset of a tachyarrhythmia based on a degree of randomness in the heart rate of the patient. The degree of randomness is determined, for example, by calculating the degree of entropy, chaos dimensionality, or spectral coefficient associated with the heart rate. If the degree of randomness falls below a predetermined threshold value, there is a significant risk of onset of a tachyarrhythmia and appropriate prophylactic steps are taken, such as aggressive overdrive pacing.
In accordance with a second aspect of the invention, a technique is provided for controlling overdrive pacing based, in part, on the degree of randomness in the heart rate of the patient. In one example, if the degree of randomness falls below a predetermined threshold value, the aggressiveness of overdrive pacing that has already begun is increased, either by raising the overdrive pacing rate, raising the overdrive pacing dwell time, or lowering the overdrive pacing rate recovery decrement. In this manner, a potential tachyarrhythmia may be avoided.
In accordance with a third aspect of the invention, a technique is provided for increasing the degree of randomness within the heart rate by chaotic pacing. In one example, pacing parameters are controlled so as to continuously vary intervals between consecutive P-waves to thereby increase the overall degree of randomness in the heart rate in an effort to prevent the heart from lapsing into a coherent state in which a tachyarrhythmia is likely.